This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This is an NIH/NIA-funded R21 study. Functional brain imaging data of patients at risk for Alzheimer's disease (by virtue of being carriers of the apoliproprotein e4 allele, APOE e4) has been shown to differ from controls when studied prior to onset of the disease. At the same time there is evidence suggesting that physical exercise reduces the risk of late onset Alzheimer's disease, especially in older APOE e4 carriers. Based on these findings the investigators plan to study subjects, using several MRI sequences for the purpose of developing a grant proposal. This grant proposal seeks to determine whether physical activity reduces the risk of late onset Alzheimer's in middle aged men and women who are carriers of APOE e4 and whether it attenuates abnormalities in neural processes that may be indicative of preclinical Alzheimer's Disease. All participants will be selected from a subgroup of participants that have participated in a University of Maryland College Park and that has already completed behavioral testing, genotyping and magnetoencephalography (MEG) studies of brain activity.